Method and device for reducing heavy polycyclic aromatic compounds in hydrocracking units

ABSTRACT

The invention concerns a process and a facility for reducing the concentration of heavy polycyclic aromatic compounds (HPNA) in the recycle loop of hydrocracking units, which comprises a fractionation column. 
     In accordance with this process, a portion of the stream present at the level of at least one plate located between the plate for supplying hydrocracked effluent and the plate for withdrawing the distillate fraction which is the heaviest is withdrawn from the fractionation column and at least a portion of said withdrawn stream is recycled to the column directly or after optional liquid separation, and optionally a portion of said withdrawn stream is recycled to the hydrocracking step directly or after optional gas separation.

The invention relates to a process and a device for reducing theconcentration of heavy polycyclic aromatic compounds (HPNA) in therecycle loop of hydrocracking units.

Hydrocracking processes are routinely used in refining to transformmixtures of hydrocarbons into products which can be upgraded easily.These processes may be used to transform light cuts such as gasolines,for example, into lighter cuts (LPG). However, they are more usuallyused to convert heavier feeds (such as oil cuts or heavy synthetics, forexample gas oils obtained from vacuum distillation or effluents from aFischer-Tropsch unit) into gasoline or naphtha, kerosene or gas oil.This type of process is also used to produce oils.

In order to increase the conversion of hydrocracking units, a portion ofthe unconverted feed is recycled, either to the reaction section throughwhich it has already passed, or to an independent reaction section. Thiscauses an unwanted accumulation in the recycle loop of polycyclicaromatic compounds formed in the reaction section during crackingreactions. These compounds poison the hydrocracking catalyst, whichreduces the catalytic activity as well as the cycle time. They can alsoprecipitate or be deposited in the cold parts of the unit, thusgenerating disruptions.

Thus, there is a need for improving the hydrocracking process in orderto reduce the formation of polycyclic aromatic compounds or to eliminatethem without reducing the yield of upgradeable products.

HPNA compounds are defined as polycyclic or polynuclear aromaticcompounds which thus comprise several condensed benzene nuclei or rings.They are usually known as HPA, Heavy Polynuclear Aromatics, or PNA orHPNA.

Typically, HPNAs known as heavies comprise at least 4 or even at least 6benzene rings in each molecule. The compounds with fewer than 6 rings(pyrene derivatives, for example) can be hydrogenated more easily andare thus less likely to poison the catalysts. As a consequence, we aremore particularly interested in compounds that are the mostrepresentative of families containing 6 aromatic rings or more such as,for example, coronene (a compound containing 24 carbon atoms),dibenzo(e,ghi) perylene (26 carbon atoms), naphtho[8,2,1,abc] coronene(30 carbon atoms) and ovalene (32 carbon atoms), which are the compoundswhich are the most easily identifiable and quantifiable, for example bychromatography.

The Applicant's U.S. Pat. No. 7,588,678 describes a hydrocrackingprocess with a recycle of the unconverted 380° C.+ fraction, in whichprocess the HPNA compounds are eliminated from the recycled fraction bymeans of an adsorbent. Other techniques for reducing the quantity or foreliminating the HPNA are described in the prior art for that patent suchas, for example, their reduction via a hydrogenation or theirprecipitation followed by a filtration.

The U.S. Pat. No. 4,961,839 describes a hydrocracking process forincreasing the conversion per pass using high flow rates of hydrogen inthe reaction zone, by vaporizing a large proportion of the hydrocarbonssent to the column for separating the products and by concentrating thepolycyclic aromatic compounds in a small heavy fraction which isextracted from that column. In that process, a heavy fraction iswithdrawn from the level of a plate located above the supply point andbelow the point for withdrawing the gas oil distillate; that heavyfraction is recycled to the hydrocracker. The bottom of the column(residue) is recycled directly to the fractionation column. That type oftechnique can indeed reduce the concentration of HPNA in the recycleloop to the reactor, but results in significant losses of yields andhigh costs linked to the quantities of hydrogen.

The patent applications WO 2012/052042 and WO 2012/052116 (correspondingto US-2013/0220885) describe a hydrocracking process in which the bottomof the fractionation column (residue) is stripped as a counter-currentin a stripping column. The light fraction obtained after stripping issent to the fractionation column and at least a portion of the heavyfraction obtained from stripping is purged, the other portion of thatfraction optionally being recycled to the stripping column.

Those processes have brought about improvements in the reduction ofHPNAs, but often to the detriment of the yields and costs.

The process of the invention can not only be used to concentrate thepolycyclic aromatic hydrocarbons in the unconverted fractions (residues)in order to eliminate them and reduce the quantity of residue purged inorder to increase the conversion, but also be used to improve the yieldof upgradeable products (for example by preventing over-cracking of gasoil) and/or the catalytic cycle time compared with prior art processes.The invention also has the advantage of considerably reducing thequantity of HPNA containing at least 6 aromatic rings presented to thehydrocracker and which are the most refractory to the reactionsoccurring during hydrocracking.

The process in accordance with the invention is based on positioning aside stream above the supply to the column, and below the withdrawal forthe heaviest gas oil distillate, returning all or a portion of saidwithdrawn fraction to the column, optionally separated from the liquid.This results in super-evaporation. The liquid is preferably separated bycombining a stripper with the fractionation column, which strips saidwithdrawn fraction.

More precisely, the invention concerns a process for hydrocracking anoil feed comprising at least 10% by volume of compounds boiling above340° C., comprising a hydrocracking step, optionally followed by aseparation of the gases from the hydrocracked effluent, then a step forfractionation of said effluent, which separates at least one distillateand a residue, a portion of said residue being recycled to thehydrocracking step and another portion of the residue being purged, saidfractionation step comprising a distillation in a column provided withplates, in which column:

-   -   said at least partially vaporized effluent supplies the column        over at least one supply plate,    -   said distillate is withdrawn from the level of a withdrawal        plate,    -   said residue is evacuated at an evacuation point,    -   and optionally, a stripping gas is injected at an injection        point located below the supply plate,        in which process    -   a portion of the stream present at the level of at least one        plate located between said supply plate and the plate for        withdrawing the heaviest distillate fraction is withdrawn from        the column, and    -   at least a portion of said withdrawn stream is recycled to the        column, directly or after optional liquid separation,    -   and optionally, at least a portion of said withdrawn stream is        recycled to the hydrocracking step directly or after optional        separation of the gases.

Advantageously, a portion of the stream present at the level of a platelocated above the supply plate and close to said supply plate,preferably at the level of the plate which is closest to the supplyplate, is withdrawn from the column.

Said withdrawn stream has a concentration of HPNA of less than 500 ppmby weight, preferably less than 350 ppm by weight and highly preferablyless than 200 ppm by weight. It usually has a proportion of at least 70%by weight of unconverted hydrocarbons, preferably at least 80% by weightof unconverted hydrocarbons and highly preferably at least 90% by weightof unconverted hydrocarbons.

In accordance with one embodiment of the process in accordance with theinvention, the entirety of said withdrawn stream is recycled directly tothe column.

In accordance with one embodiment of the process in accordance with theinvention:

-   -   a portion of said withdrawn stream is recycled directly (i.e.        without treatment of the withdrawn stream before recycling to        the column) to the column, for example as shown in FIG. 2 a,    -   the other portion of the withdrawn stream is recycled to the        hydrocracking step, directly or after optional separation of the        gases.

Advantageously, said stream (all or a portion) is recycled to the columnbelow the supply plate and, when it exists, above the point forinjection of the stripping gas. Highly preferably, in addition, it isrecycled to the column at the level of the plate closest to said supplyplate.

In accordance with another preferred embodiment of the process inaccordance with the invention:

-   -   all or a portion, preferably all, of said withdrawn stream is        separated in an external stripping step using a stripping gas,        and    -   all or a portion, preferably all of the separated gaseous        effluent is recycled to the column above the plate from which        the stream has been withdrawn, and preferably to the level of a        plate close to the plate from which the stream has been        withdrawn,    -   preferably, in addition, the gaseous effluent is recycled to the        column to the level of the plate closest to the plate from which        the stream has been withdrawn,    -   preferably, in addition, all or a portion, preferably all of the        liquid effluent separated in said stripping step is recycled,        preferably directly to the hydrocracking step.

Furthermore, in a preferred disposition of this embodiment, the liquidfraction separated in the stripping step is not recycled to thefractionation column.

It should also be noted that preferably, in accordance with theinvention, residue is not recycled to the fractionation step.

Preferably, the process operates in the presence of a stripping gasinjected into the fractionation step. Preferably, it is steam,preferably at a pressure in the range 0.2 to 1.5 MPa.

The stripping gas injected into the external stripping step ispreferably steam, preferably at a pressure in the range 0.2 to 1.5 MPa.

The hydrocracking step is carried out in conventional manner at atemperature of more than 200° C., a pressure of more than 1 MPa, a spacevelocity of 0.1 to 20 h⁻¹, and the H₂/hydrocarbons volume ratio is 80 to5000 NL/L.

The invention also concerns a facility which is advantageously employedin order to carry out the process in accordance with the invention.

It comprises:

-   -   a hydrocracking section 2 provided with an inlet line 1 for the        feed and an inlet line 8 for hydrogen,    -   optionally followed by a zone 4 for separating effluent (line 3)        in order to separate a gaseous fraction (line 5),    -   followed by a fractionation section 12 comprising at least one        distillation column provided with plates, said column        comprising:        -   at least one line 11 for the inflow of at least partially            vaporized hydrocracked effluent onto at least one supply            plate,        -   at least one line 14 for withdrawing at least one distillate            from the level of a withdrawal plate,        -   at least one line 15 a for evacuating residue,    -   at least one line 23, 18 for recycling a portion of the residue        to the hydrocracking step,    -   and optionally comprising at least one line 19 for injecting a        stripping gas, the injection point being located below the        supply plate,        the facility further comprising:    -   at least one line 20 for withdrawing a portion of the stream        present at the level of at least one plate located between said        supply plate and the plate for withdrawing the heaviest        distillate fraction (i.e. above the supply plate),    -   and a line for recycling all or a portion of said withdrawn        stream to said column, directly (line 22 a) or after optional        liquid separation (line 22 b),    -   and optionally, a line for recycling all or a portion of said        withdrawn stream to the hydrocracking section, directly (line 17        a) or after an optional gas separation (line 17 b).

Preferably, the plate for withdrawing said stream from the line 20 ispositioned at the level of a plate close to the supply plate. Highlypreferably, the plate for withdrawing said stream from the line 20 ispositioned at the level of the plate closest to the supply plate.

In one embodiment, the facility does not comprise treatment of thestream withdrawn before recycling to the column; it comprises a line 22a for recycling the entirety of the withdrawn stream directly to thecolumn (i.e. no treatment of the withdrawn stream before recycling tothe column).

In one embodiment, the facility does not comprise a treatment of thestream withdrawn before recycling to the column, it comprises:

-   -   a line 22 a for recycling a portion of the withdrawn stream        directly to the column, and    -   a line 17 a for recycling a portion of the withdrawn stream        directly to the hydrocracking section 2,    -   advantageously, the line 22 a discharges at the level of a plate        located below the supply plate and, if it exists, above the        stripping gas injection point 19, and preferably to a plate        close to the supply plate,    -   preferably, the line 22 a discharges at the level of a lower        plate closest to the supply plate.

In another preferred embodiment, the facility in accordance with theinvention comprises:

-   -   a stripper 25 external to the column, provided with an inlet        line 20 for said withdrawn stream, a stripping gas injection        line 26, an outlet line 22 b for the gaseous fraction, an outlet        line 17 b for the liquid fraction,    -   a line 22 b for recycling all or a portion, preferably all, of        said gaseous fraction to said column,    -   a line 17 b, 18 for recycling all or a portion, preferably all,        of said liquid fraction to the hydrocracking step,    -   a line 16 to purge a portion of the residue,    -   preferably, the recycling line 22 b discharges into the column        above the plate from which the stream has been withdrawn, and        preferably to the level of a plate close to the plate from which        the stream has been withdrawn, and preferably to the level of        the plate closest to the plate from which the stream has been        withdrawn.

In a preferred disposition of this embodiment, there is no line forrecycling a liquid fraction separated in the stripping step towards thefractionation column.

Preferably, the facility in accordance with the invention does notcomprise a line for recycling residue to the column.

In the text, feeds are defined by their T5 boiling point (as will beexplained below). The conversion of the feed is defined with respect tothe cut point of the residue. The unconverted fraction is termedresidue. The converted fraction comprises the fractions sought by therefiner (objectives).

The invention will be better understood from the following descriptionof the figures.

The purged portion refers to a portion which leaves the process.

FIG. 1 represents the prior art. FIGS. 2a and 2b represent theinvention. The principle of the invention will be explained commencingfrom FIG. 2a . FIGS. 2a and 2b will be understood in combination withFIG. 1, and more precisely with the essential elements of FIG. 1 citedin the claims.

FIG. 1 presents a flowchart for a prior art hydrocracking process. Tofacilitate reading, the description of the conditions employed has beenmoved to a further part of the text below.

The feed (line 1) composed of hydrocarbons of oil origin and/orsynthetic hydrocarbons with a mineral or biological source is mixed withhydrogen supplied via the lines 5 (recycle) and/or 6 (makeup hydrogen)via the compressor 7 and the line 8. The feed/hydrogen mixture thusformed is sent to the hydrocracking section 2. This section comprisesone or more fixed bed or ebullated bed reactors.

When the hydrocracking section comprises one or more fixed bed reactors,each reactor may comprise one or more beds of catalyst carrying outhydrocracking of the hydrocarbons of the feed to form lighterhydrocarbons.

When the hydrocracking section comprises one or more ebullated bedreactors, a stream comprising liquid, solid and gas moves verticallythrough a reactor containing a bed of catalyst. The catalyst in the bedis maintained in a random motion in the liquid. The gross volume of thecatalyst dispersed through the liquid is thus larger than the volume ofcatalyst when stopped. This technology has been widely described in theliterature.

A mixture of liquid hydrocarbon and hydrogen is passed through the bedof particles of catalyst at a velocity such that the particles arecaused to move in a random manner and thus become suspended in theliquid. Expansion of the catalytic bed in the liquid phase is controlledby the flow rate of recycle liquid in a manner such that in theequilibrium state, the major portion of catalyst does not go above adefined level in the reactor. The catalysts are in the form ofextrudates or beads, preferably with a diameter in the range 0.8 mm to6.5 mm in diameter.

In an ebullated bed process, large quantities of hydrogen gas and lighthydrocarbon vapours rise through the reaction zone then in a zone whichis free of catalyst. A portion of the liquid from the catalytic zone isrecycled to the bottom of the reactor after separating a gaseousfraction and a portion is withdrawn from the reactor as product, usuallyat the top portion of the reactor.

The reactors used in an ebullated bed process are generally designedwith a central vertical recycling conduit which acts as a flow tube forrecycling liquid from the catalyst-free zone located above the ebullatedbed of catalyst, via a recycling pump which can be used to recycle theliquid in the catalytic zone. Recycling the liquid means that both auniform temperature can be maintained in the reactor and that the bed ofcatalyst can be kept in suspension.

The hydrocracking section may be preceded by or include one or more bedsof hydrotreatment catalyst(s).

The effluent from the hydrocracking section 2 is sent via line 3 to aseparation zone 4 in order to recover a gaseous fraction 5 on the onehand, along with a liquid fraction 9. The gaseous fraction 5 containsexcess hydrogen which has not reacted in the reaction section 2. It isgenerally combined with fresh hydrogen arriving via the line 6 in orderto be recycled as indicated below.

The liquid fraction 9 is reheated by any means 10, for example a furnacewhich could be associated with an exchanger (not shown), in order to atleast partially vaporize it before supplying the fractionation section12 via the line 11.

The fractionation section 12 comprises one or more distillation columnsequipped with plates and contact means in order to separate variousupgradeable cuts (distillates) which are withdrawn by means of the lines13 and 14, plus other optional side streams. These cuts have boilingpoint ranges situated, for example, in the gasoline, kerosene and gasoil ranges.

A heavier unconverted fraction (residue) is recovered from the bottom ofthe column (line 15 a).

Stripping gas may be injected via the line 19. This line is locatedbetween the plate for supplying hydrocracked effluent (line 11) and theresidue evacuation point (line 15 a).

A portion of the residue may be purged via the line 16, with anotherportion recycled to the hydrocracking section via the lines 23 and 18and another portion recycled to the fractionation section (line 15 b).

In accordance with FIG. 1, a portion (line 15 b) of the residue from theline 15 a is mixed with the supply (line 9) upstream of the furnace 10of the fractionation section and recycled as a mixture towards thefractionation section (line 11).

The purge 16 can in particular be used to eliminate at least a portionof the HPNA compounds which could accumulate in the recycle loop withoutthis purge.

The zone E outlined in FIG. 1 defines the portion modified by thesubject matter of the present invention.

FIGS. 2a and 2b present the invention.

The elements described above will not be described again here. It willbe noted that the line 15 b (recycle of residue to the fractionationcolumn) is preferably dispensed with in the invention.

The fractionation section 12 comprises a single fractionation column.However, the invention could be implemented with several fractionationcolumns and at least one column would then comprise a zone E inaccordance with the invention.

In accordance with FIG. 2a , the liquid fraction 11 which has previouslybeen at least partially vaporized is supplied to the fractionationsection 12.

Preferably, a stripping gas is injected into the column (line 19).Advantageously, it is steam, preferably low pressure steam, preferablyat a pressure in the range 0.2 to 1.5 MPa (0.1 MPa=1 bar or 1 barg). Theinjection point is located below the supply plate and above the residueevacuation point. It is preferably close to the point for evacuation ofresidue from the bottom of the column.

FIG. 2a differs from FIG. 1 primarily in that a side stream is added(line 20) at one of the plates of the column. It is possible to positionone or more side streams at the level of the column. Thus, a portion ofthe stream present at the level of at least one plate located betweenthe plate for supplying effluent and the plate (line 14) for withdrawingthe heaviest distillate fraction is withdrawn.

This withdrawal (line 20) is preferably close to the supply plate.Preferably, a portion of the stream present at the level of the plateclosest to the supply plate is withdrawn from the column.

All or a portion of said withdrawn stream is recycled directly to thecolumn (line 22 a).

FIG. 2b (described below) describes another embodiment in whichrecycling to the column is carried out after separation of the liquid.

FIG. 2a presents the case in which a portion of the side stream (line20) is recycled to the column (line 22 a), the other portion of the sidestream (line 17 a) being recycled (line 18) to the hydrocracking step,preferably as a mixture with the portion of the residue recycled (line23) to the hydrocracking step.

The side stream (line 20) is positioned in a manner such that thewithdrawn stream has a low concentration of HPNA of less than 500 ppm byweight, preferably less than 350 ppm by weight and highly preferablyless than 200 ppm by weight, and most often, a large proportion ofunconverted hydrocarbons in the hydrocracking section of at least 70% byweight of unconverted hydrocarbons, preferably at least 80% by weight ofunconverted hydrocarbons and highly preferably at least 90% by weight ofunconverted hydrocarbons.

In order to satisfy these criteria, the side stream (line 20) ispreferably positioned above the supply plate, and preferably at thelevel of the plate closest to the supply plate.

In this embodiment (such as FIG. 2a ), the withdrawn stream isre-injected below the supply plate and, when it exists, above the pointfor injecting stripping gas, and preferably to the level of the plateclosest to the supply plate.

The reference numerals in FIGS. 1 and 2 a will not be described again inthe description of FIG. 2b . FIG. 2b differs from FIG. 2a in respect ofthe treatment and recycling of the withdrawn stream.

In accordance with FIG. 2b , all or a portion of said withdrawn streamis recycled to the column after separating the liquid.

Preferably, the withdrawn stream (line 20) is stripped in an externalstripping step (stripper 25) by a stripping gas (supplied via the line26). All or a portion of the separated gaseous effluent is recycled tothe column; in accordance with FIG. 2b , all of the gaseous effluent isrecycled (22 b).

The side stream (line 20) is positioned in the same manner as in FIG. 2a.

Preferably, the gaseous effluent is recycled to the column above theplate from which the stream has been withdrawn. In addition, betterperformances are obtained when the gaseous effluent is recycled to thecolumn to the plate closest to the plate from which the stream has beenwithdrawn.

All or a portion of the liquid effluent (line 17 b) is recycled, usuallydirectly, to the hydrocracking step. In accordance with FIG. 2b , all ofthe liquid effluent is mixed with the recycled residue portion (line 23)and the mixture is recycled (line 18) to the hydrocracking step.

Said lateral stripper 25 functions with injection of a stripping gas(line 26). This gas is preferably steam, preferably low pressure steam,preferably at a pressure in the range 0.2 to 1.5 MPa.

As will be demonstrated in the examples below, the embodiment of FIG. 2bresults in better performances than the embodiment of FIG. 2 a.

Description of the Conditions for the Hydrocracking, 2, and SeparationSteps

This description refers to conventional implementational conditionswhich can be applied both to FIG. 1 (prior art) and to the invention(FIGS. 2a and 2b ).

Feeds:

A wide variety of feeds may be treated in hydrocracking processes. Ingeneral, they contain at least 10% by volume, generally at least 20% byvolume and often at least 80% by volume of compounds boiling above 340°C.

The feed may, for example, be LCOs (light cycle oil—light gas oilsobtained from a catalytic cracking unit), atmospheric distillates,vacuum distillates, for example gas oils obtained from straight rundistillation of crude or from conversion units such as FCC, coking orvisbreaking, as well as feeds originating from units for the extractionof aromatics from lubricating oil bases or obtained from solventdewaxing of lubricating base oils, or in fact from distillatesoriginating from processes for fixed bed or ebullated bedhydroconversion or desulphurization of AR (atmospheric residues) and/orVR (vacuum residues) and/or deasphalted oils, or the feed may in fact bea deasphalted oil, effluents from a Fischer-Tropsch unit or in fact anymixture of the feeds cited above. The above list is not limiting.

In general, the feeds have a T5 boiling point of more than 150° C. (i.e.95% of the compounds present in the feed have a boiling point of morethan 150° C.). In the case of gas oil, the T5 point is generallyapproximately 150° C. In the case of VGO, the T5 is generally more than340° C., or even more than 370° C. The feeds which may be used thus fallwithin a wide range of boiling points. This range generally extends fromgas oil to VGO, encompassing all possible mixtures with other feeds, forexample LCO.

The nitrogen content of the feeds treated in the hydrocracking processesis usually more than 500 ppm by weight, generally in the range 500 to10000 ppm by weight, more generally in the range 700 to 4500 ppm byweight and still more generally in the range 800 to 4500 ppm by weight.

The sulphur content in the feeds treated in the hydrocracking processesis usually in the range 0.01% to 5% by weight, generally in the range0.2% to 4% by weight and yet more generally in the range 0.5% to 3% byweight. The feed may optionally contain metals. The cumulative nickeland vanadium content in the feeds treated in hydrocracking processes ispreferably less than 10 ppm by weight, preferably less than 5 ppm byweight and yet more preferably less than 2 ppm by weight. Theasphaltenes content is generally less than 3000 ppm by weight,preferably less than 1000 ppm by weight, and yet more preferably lessthan 300 ppm by weight.

Guard Beds

In the case in which the feed contains compounds of the resins and/orasphaltenes type, it is advantageous to initially pass the feed over abed of catalyst or adsorbent which differs from the hydrocracking orhydrotreatment catalyst. The catalysts or guard beds used are in theshape of spheres or extrudates. Any other shape may be used. Particularpossible shapes which may be used are included in the followingnon-limiting list: hollow cylinders, hollow rings, Raschig rings,toothed hollow cylinders, crenelated hollow cylinders, wheels known aspentarings, multiple-holed cylinders, etc. These catalysts may have beenimpregnated with a phase which may or may not be active. Preferably, thecatalysts are impregnated with a hydrodehydrogenating phase. Highlypreferably, the CoMo or NiMo phase is used. These catalysts may have amacroporosity.

Operating Conditions:

The operating conditions such as temperature, pressure, hydrogen recycleratio, or hourly space velocity may vary widely as a function of thenature of the feed, the quality of the desired products and thefacilities available to the refiner. The hydrocracking/hydroconversioncatalyst or hydrotreatment catalyst is generally brought into contactwith the feeds described above in the presence of hydrogen, at atemperature of more than 200° C., often in the range 250° C. to 480° C.,advantageously in the range 320° C. to 450° C., preferably in the range330° C. to 435° C., at a pressure of more than 1 MPa, often in the range2 to 25 MPa, preferably in the range 3 to 20 MPa, the space velocitybeing in the range 0.1 to 20 h⁻¹, preferably in the range 0.1 to 6 h⁻¹,and more preferably in the range 0.2 to 3 h⁻¹, and the quantity ofhydrogen introduced being such that the volume ratio in litres ofhydrogen/litres of hydrocarbon is in the range 80 to 5000 NL/L, usuallyin the range 100 to 3000 NL/L.

These operating conditions used in the hydrocracking processes cangenerally be used to obtain conversions per pass into converted products(i.e. with boiling points below the residue cut point) of more than 15%,and more preferably in the range 20% to 95%.

The Principal Aims:

The invention may be used in all hydrocrackers, namely:

-   -   the maxi-naphtha hydrocracker with a residue cut point which is        generally between 150° C. and 190° C., preferably between        160° C. and 190° C., and usually 170° C.-180° C.,    -   the maxi-kerosene hydrocracker with a residue cut point which is        generally between 240° C. and 290° C., and usually 260° C.-280°        C.,    -   the maxi-gas oil hydrocracker with a residue cut point which is        generally between 340° C. and 385° C., and usually 360° C.-380°        C.

Embodiments

The hydrocracking/hydroconversion processes using the catalysts inaccordance with the invention cover the ranges of pressure andconversion from mild hydrocracking to high pressure hydrocracking.

The term “mild hydrocracking” means hydrocracking resulting in moderateconversions, generally below 40%, and operating at low pressures,generally between 2 MPa and 9 MPa. The hydrocracking catalyst may beused alone, in a single or in more fixed bed catalytic beds, in one ormore reactors, in a “once-through” hydrocracking layout, with or withoutliquid recycling of the unconverted fraction, optionally in associationwith a hydrorefining catalyst located upstream of the hydrocrackingcatalyst.

The hydrocracking may be operated at high pressure (at least 10 MPa).

In a first variation, the hydrocracking may be operated in accordancewith a hydrocracking layout which is known as a “two-step” layout, withintermediate separation between the two reaction zones; in a given step,the hydrocracking catalyst may be used in one or in both reactorsassociated or otherwise with a hydrorefining catalyst located upstreamof the hydrocracking catalyst.

In a second variation, what is known as “once-through” hydrocracking maybe carried out. This variation generally initially comprises intensehydrorefining which is intended to carry out intensehydrodenitrogenation and hydrodesulphurization of the feed before it issent to the hydrocracking catalyst proper, in particular in the case inwhich it comprises a zeolite. This intense hydrorefining of the feedbrings about only a limited conversion of this feed into lighterfractions. The conversion, which is still insufficient, must thereforebe supplemented on the more active hydrocracking catalyst.

The hydrocracking section may contain one or more beds of identical ordifferent catalysts. When the preferred products are middle distillates,basic amorphous solids are used, for example alumina or silica-aluminasor basic zeolites, optionally supplemented with at least onehydrogenating metal from group VIII and preferably also supplementedwith at least one metal from group VIB. These basic zeolites arecomposed of silica, alumina and one or more exchangeable cations such assodium, magnesium, calcium or rare earths.

When gasoline is the major desired product, the catalyst is generallycomposed of a crystalline zeolite onto which small quantities of a metalfrom group VIII are deposited, and also, more preferably, a metal fromgroup VIB.

The zeolites which may be used are natural or synthetic and may, forexample, be selected from X, Y or L zeolites, faujasite, mordenite,erionite or chabasite.

Hydrocracking may be carried out in just one or in more ebullated bedreactors, with or without a liquid recycle of the unconverted fraction,optionally in association with a hydrorefining catalyst located in afixed bed or ebullated bed reactor upstream of the hydrocrackingcatalyst. The ebullated bed is operated with withdrawal of spentcatalyst and the daily addition of fresh catalyst in order to keep thecatalyst activity stable.

Liquid/Gas Separation (4):

The separator 4 separates the liquid and gas present in the effluentleaving the hydrocracking unit. Any type of separator that can carry outthis separation may be used, for example a flash drum, a stripper, oreven a simple distillation column.

Fractionation (12):

The fractionation section is generally constituted by one or morecolumns comprising a plurality of plates and/or internal packing whichmay preferably be operated in counter-current mode. These columns areusually steam stripped and include a reboiler in order to facilitatevaporization. It can be used to separate hydrogen sulphide (H₂S) andlight components (methane, ethane, propane, butane etc) from theeffluents, as well as the hydrocarbon cuts with boiling points in thegasoline, kerosene and gas oil ranges along with a heavy fractionrecovered from the bottom of the column, all or a portion of which maybe recycled to the hydrocracking section.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the prior art.

FIGS. 2a, and 2b are schematic representations of processes of theinvention.

EXAMPLES Example 1: Prior Art

This example is based on the configuration of FIG. 1. Two samples froman operating industrial unit based on the configuration of FIG. 1 wereanalysed. The properties are recorded in Table 1 below.

It should be noted that because of the configuration, the streams 15 a,16, 18 and 23 had exactly the same properties.

The fractionation of stream 11 in the column 12 was computer simulatedusing the PRO/II version 8.3.3 software marketed by SimSci. The physicaland analytical properties of the resulting streams were simulated andcompared with the physical and analytical properties of actual samples.

The operating conditions for the column used for the simulation arerecorded in Table 2 below.

TABLE 1 Properties of the streams of the layout of FIG. 1 Streams fromFIG. 1 Stream number Configuration 11 15a 18 16 Yield % by wt 100 4239.5 2.5 Quantity of gas % by wt 64.0 10.9 10.9 10.9 oil in streamDensity 0.805 0.828 0.828 0.828 HPNA Coronene ppm by wt 209 497 497 497Dibenzo(e,ghi)per- ppm by wt 33 78 78 78 ylene Naphtho[8,2,1 abc] ppm bywt 81 192 192 192 coronene Ovalene ppm by wt 57 135 135 135 Total HPNAppm by wt 378 902 902 902 TBP, % by wt Initial boiling point ° C. 128200 200 200 10% ° C. 200 368 368 368 50% ° C. 326 402 402 402 90% ° C.440 477 477 477 Final boiling point ° C. 524 524 524 524 1: Specificgravity SG = ρ_(sample) at 20° C./ρ_(H20) at 4° C., where ρ is thedensity expressed in g/cm³

Starting from the properties of the stream 11 entering the fractionationcolumn (see Table 1), the PRO/II simulation was able to establish theproperties of the stream 15 leaving the fractionation column; inparticular, the HPNA distribution could be modelled.

TABLE 2 Operating conditions for the column Operating conditions forfractionation FIG. 1 Pressure, top of column Barg 1.0 Pressure, bottomof column Barg 1.5 Temperature, inlet feed ° C. 377 Number oftheoretical plates 34 Flow rate of stripping steam kg of steam/tonne offeed 17

Based on these results, the configurations of the invention weresimulated. The results are disclosed below for each configuration.

Example 2: Configuration 2a

Table 3 below provides the characteristics of the streams 11, 16 and 18in the configuration 2a obtained from the PRO/II simulation. Theoperating conditions used for the simulation are recorded in Table 4.

TABLE 3 Properties of the streams of the layout of FIG. 2a Streams fromFIG. 2a Stream number 11 18 16 Configuration inlet liquid recycle purgeYield 100 39.5 2.5 Quantity of gas oil in stream 64.0 14.8 5.1 Density0.805 0.8275 0.832 HPNA Coronene 209 422 2192 Dibenzo(e,ghi)perylene 3396 314 Naphtho [8,2,1 abc] coronene 81 114 895 Ovalene 57 70 640 TotalHPNA 378 702 4041 TBP, % by wt Initial boiling point 128 79 251 10% 200362 389 50% 326 401 425 90% 440 476 505 Final boiling point 524 524 5241: Specific gravity SG = ρ_(sample) at 20° C./ρ_(H20) at 4° C., where ρis the density expressed in g/cm³

TABLE 4 Operating conditions for the column Operating conditions forfractionation FIG. 2a Pressure, top of column Barg 1.0 Pressure, bottomof column Barg 1.5 Temperature, inlet feed ° C. 377 Number oftheoretical plates 34 Flow rate of stripping steam kg of steam/tonne offeed 17

Compared with the configuration of FIG. 1, the configuration 2a can beused to maximize the quantity of HPNA (4041 ppm by weight compared with902 ppm by weight in configuration 1) in the unconverted fraction(residue) which was purged via the line 16. At the same time, thequantity of HPNA was minimized in the stream which returns to thereaction section via the line 18: 702 ppm by weight (by wt) comparedwith 902 ppm by weight in configuration 1, which reduced the quantity ofHPNA by 22.2%.

In addition, the proportion of heavy HPNA (naphtho [8,2,1 abc]coronene+ovalene) compared with the total quantity of HPNA in the stream18 returning to the reaction section was much lower for theconfiguration 2a (26.3%) than for the configuration 1 (36.3%). Thisindicates that not only was there less total HPNA in the streamreturning to the reaction section via the line 18, but also, theproportion of the most refractory and poisonous heavy HPNA (naphtho[8,2,1 abc] coronene+ovalene) was lower.

Example 3: Configuration 2b

Table 5 below provides the characteristics obtained from the PRO/IIsimulation for the streams 11, 16 and 18 in the configuration 2b. Theoperating conditions used for the simulation are recorded in Table 6.

TABLE 5 Properties of the streams of the layout of FIG. 2b Streams fromFIG. 2b Stream number 11 18 16 Configuration inlet liquid recycle purgeYield 100 39.5 2.5 Quantity of gas oil in stream 64.0 6.6 5.7 Density0.805 0.8291 0.8313 HPNA Coronene 209 388 2500 Dibenzo(e,ghi) perylene33 78 375 Naphtho [8,2,1 abc] coronene 81 122 994 Ovalene 57 80 704Total HPNA 378 668 4573 TBP, % by wt Initial boiling point 128 217 21010% 200 384 380 50% 326 401 421 90% 440 476 504 Final boiling point 524524 524 1: Specific gravity SG = ρ_(sample) at 20° C./ρ_(H20) at 4° C.,where ρ is the density expressed in g/cm³

TABLE 6 Operating conditions for the column FIG. 2b Operating conditionsfor fractionation Pressure, top of column Barg 1.0 Pressure, bottom ofcolumn Barg 1.5 Temperature, inlet feed ° C. 377 Number of theoreticalplates 34 Flow rate of stripping steam kg of steam/tonne of feed 17operating conditions for side stripper Pressure, top of column Barg 1.4Pressure, bottom of column Barg 1.5 Number of theoretical plates 6 Flowrate of stripping steam kg of steam/t of feed 28

Compared with FIG. 1, the configuration 2b can be used to maximize thequantity of HPNA: 4573 ppm by weight compared with 902 ppm by weight forconfiguration 1 in the unconverted fraction (residue) which was purgedvia the line 16. At the same time, the quantity of HPNA was minimized inthe stream leaving the reaction section via the line 18: 668 ppm byweight compared with 902 ppm by weight for configuration 1, whichreduced the quantity of HPNA by 25.9%.

In addition, the proportion of the most refractory and poisonous heavyHPNA (naphtho [8,2,1 abc] coronene+ovalene) compared with the totalquantity of HPNA in the stream 18 returning to the reaction section wasmuch lower for configuration 2b (30.3%) than for configuration 1(36.3%). This indicates that not only was there less total HPNA in thestream returning to the reaction section via the line 18, but also thatthe proportion of heavy HPNA (naphtho [8,2,1 abc] coronene+ovalene) waslower.

This configuration could also minimize the quantity of gas oil sent tothe reaction section via the line 18 because the quantity of gas oilincluded in the stream which was sent to the reaction section via theline 18 was only 6.6% by weight compared with 10.9% by weight inconfiguration 1.

The invention claimed is:
 1. A process for hydrocracking an oil feedcomprising at least 10% by volume of compounds boiling above 340° C.,comprising hydrocracking said oil feed to produce a hydrocrackedeffluent, followed by an optional separation of gases from thehydrocracked effluent, then fractionation of said effluent, separatingat least one distillate and a residue, a portion of said residue beingrecycled to hydrocracking and another portion of the residue beingpurged, said fractionation comprising a distillation in a columnprovided with plates, in which column: at least partially vaporizedhydrocracked effluent supplies the column over at least one supplyplate, said distillate is withdrawn from the level of a withdrawalplate, said residue is evacuated at an evacuation point, and optionally,a stripping gas is injected at an injection point located below thesupply plate, in which process a portion of a stream present at thelevel of at least one plate located between the supply plate and a platefor withdrawing a heaviest distillate fraction is withdrawn from thecolumn, a first portion of said withdrawn stream is recycled to thecolumn, directly or after optional liquid separation, and anotherportion of the withdrawn stream is recycled to the hydrocracking,directly or after optional separation of gases.
 2. The process asclaimed in claim 1, in which the withdrawn stream comprises a portion ofa stream present at the level of a plate located above the supply plateand at the level of the plate which is closest to the supply plate. 3.The process as claimed in claim 1, in which said withdrawn stream has aconcentration of HPNA of less than 500 ppm by weight.
 4. The process asclaimed in claim 1, in which said withdrawn stream has a proportion ofat least 70% by weight of unconverted hydrocarbons.
 5. The process asclaimed in claim 1, in which the entirety of said first portion of saidwithdrawn stream is recycled directly to the column.
 6. The process asclaimed in claim 1, in which said first portion of said withdrawn streamis recycled to the column above the supply plate and, when it exists,above the point for injection of the stripping gas.
 7. The process asclaimed in claim 6, in which said first portion of said withdrawn streamis recycled to the column at the level of the plate closest to saidsupply plate.
 8. The process as claimed in claim 1, in which all or aportion of said withdrawn stream is stripped in an external strippingstep using a stripping gas, creating a gaseous effluent and all or aportion of the gaseous effluent separated is recycled to the columnabove the plate from which the stream has been withdrawn.
 9. The processas claimed in claim 8, in which the gaseous effluent is recycled to thecolumn at the level of the plate closest to the plate from which thestream has been withdrawn.
 10. The process as claimed in claim 8, inwhich all or a portion of the liquid effluent separated in saidstripping step is recycled directly to the hydrocracking.
 11. Theprocess as claimed in claim 8, in which the stripping gas is injectedinto the fractionation.
 12. The process as claimed in claim 11, in whichthe stripping gas injected into the fractionation is steam.
 13. Theprocess as claimed in claim 8, in which the stripping gas injected intothe external stripping step is steam.
 14. The process as claimed inclaim 1, which does not comprise recycling the residue to saidfractionation.